Why can't I open the side menu in 2.8?
This seems to be a bug with the most recent builds of Blender 2.8 (as of writing, build of April 26, 07:03:52 - 513b71c89ad0). You can open these items via the shortcuts N and T for the right and left context menus respectively. Once you toggle the menus via these shortcuts they will respond to the mouse correctly thereafter.

In case these aren't your keybindings (these should be Blender defaults), you can find and/or map them in Edit>Preferences>Keymap. You can search "context toggle" by name or find them under the keymaps of the respective editor windows. You are mostly looking for "generic" editor modes NOTE: Every editor window (3D view, node editor, image editor, etc.) has an independent keymap for this action which means you will have to change every case of this mapping you find if they are all not to your liking.